Quantum Cascade Lasers and the Future of Medical Diagnostics

Princeton scientists have developed a new more energy efficient way to generate laser light. The finding should allow for building of miniature diagnostic devices that monitor light’s attenuation in a liquid sample, potentially leading to specialized blood glucose meters and other similar devices.

The laser used in the Princeton study is a special type called a quantum cascade laser. Built at Princeton University’s nanofabrication facility, the device is about one-tenth as thick as a human hair and 3 millimeters long. Despite its tiny size, it is made of hundreds of layers of different semiconductor materials. Each layer is only a few atoms thick. In this device, electrons “cascade” down through the layers as they lose energy and give off synchronized photons.
In an earlier study published in Applied Physics Letters in June 2007, Franz [graduate student Kale Franz], Gmachl [Claire Gmachl, an electrical engineer and director of the Mid-Infrared Technologies for Health and the Environment (MIRTHE) center –ed.] and others had reported that a quantum cascade laser they had built unexpectedly emitted a second laser beam of slightly smaller wavelength than the main one. Further studies by Menzel and others revealed that the second beam could not be explained by any existing theory of quantum cascade lasers. Unlike a conventional semiconductor laser, the second beam grew stronger as the temperature increased, up to a point. Further, it seemed to compete with the “normal” laser, growing weaker as the latter strengthened when more electric current was supplied. “It’s a new mechanism of light emission from semiconductor lasers,” said Franz.
To explain this mechanism, the researchers invoked a quantum property of electrons called momentum. In the conventional view of quantum cascade lasers, only electrons of nearly zero momentum participate in “lasing” (producing laser light). Further, a substantial number of electrons has to attain the same level of energy and momentum â€“ be in a so-called “quasi-equilibrium” condition — before they can participate in laser action. In contrast, studies by Gmachl’s group showed that the second laser beam originated from electrons of lower energy, but higher momentum that were not in equilibrium. “It showed, contrary to what was believed, that electrons are useful for laser emission even when they are in highly non-equilibrium states,” said Franz.
The new laser phenomenon has some interesting features. For instance, in a conventional laser relying on low momentum electrons, electrons often reabsorb the emitted photons, and this reduces overall efficiency. In the new type of laser, however, this absorption is reduced by 90%, said Franz. This could potentially allow the device to run at lower currents, and also makes it less vulnerable to temperature changes. “It should let us dramatically improve laser performance,” he said.
The device used in the study does not fully attain this level of performance, because the conventional, low-efficiency laser mechanism dominates. To take full advantage of the new discovery, therefore, the conventional mechanism would need to be turned off. The researchers have started to work on methods to achieve this outcome, said Franz.
Unlike other lasers, quantum cascade lasers operate in the mid- and far-infrared range, and can be used to detect even minute traces of water vapor, ammonia, nitrogen oxides, and other gases that absorb infrared light. As a result, these devices are finding applications in air quality monitoring, medical diagnostics, homeland security, and other areas that require extremely sensitive detection of different chemicals.